The invention is related to short-arc high intensity discharge lamps such as used in certain photographic projectors. More particularly, the invention is related to an arc tube for short-arc high intensity discharge lamps containing a dosage that increases the lumens per watt or efficacy output and extends the anticipated life both for the arc tube.
A typical short-arc high intensity discharge lamp has an arc tube which comprises a quartz envelope having a thick-wall bulb like arc chamber, and a pair of elongated electrodes sealed into the stems of the envelope and extending from the envelope at opposite ends along a common axis. The arc chamber between the inner ends of the electrodes is about 2 to 3 millimeters, the overall bulb diameter is about 8 to 10 millimeters, and the overall length (including stems) is about 5 centimeters for a 300-watt short-arc high intensity discharge lamp.
U.S. Pat. No. 4,138,621 discloses such a short-arc discharge lamp mounted in a reflector. U.S. Pat. No. 3,780,342 to Grimshaw et al discloses a ballast circuit for the short-arc lamp, which applies a relatively high starting voltage pulse to the lamp electrodes, followed by a relatively low operating voltage.
Although the short-arc discharge lamp of U.S. Pat. No. 4,138,621 serves its desired purpose, it is desired that the efficacy, the anticipated life and total light all of the lamp be further increased. The efficacy and anticipated life are primarily determined by the arc tube of the short-arc discharge lamp.
Accordingly, objects of the present invention provide an arc tube for the short-arc high intensity discharge lamp having increased efficacy as well as increased anticipated life.